k2gl/dsse

Sign and verify DSSE (Dead Simple Signing Envelope) payloads in PHP with pluggable keys. It's the envelope Sigstore, in-toto, SLSA and npm provenance use to wrap a signed payload.

It gives you the three pieces of the spec and nothing else:

• PAE — the exact, binary-safe byte string that gets signed.
• Envelope — the JSON envelope (payload / payloadType / signatures) with lossless (de)serialization.
• Signer / Verifier — tiny interfaces so you can plug in any key (or a remote KMS/HSM). ECDSA (P-256/384/521), Ed25519, and RSA (PKCS#1 v1.5) implementations are included.

Install

composer require k2gl/dsse

Requires PHP 8.1+. The bundled signers use ext-openssl (ECDSA P-256/384/521, RSA) and ext-sodium (Ed25519); both ship with PHP by default. The core (Pae, Envelope) needs neither.

Usage

PAE — what actually gets signed

use K2gl\Dsse\Pae;

Pae::encode('http://example.com/HelloWorld', 'hello world');
// "DSSEv1 29 http://example.com/HelloWorld 11 hello world"

Lengths are byte counts, so the encoding is unambiguous for any payload, including binary data.

Sign

use K2gl\Dsse\Envelope;
use K2gl\Dsse\EcdsaP256Signer;

$signer   = EcdsaP256Signer::fromPem($privateKeyPem, keyId: 'k1');
$envelope = Envelope::sign('hello world', 'http://example.com/HelloWorld', $signer);

echo $envelope->toJson();
// {"payload":"aGVsbG8gd29ybGQ=","payloadType":"http://example.com/HelloWorld","signatures":[{"keyid":"k1","sig":"..."}]}

Envelope::sign() accepts several signers to produce a multi-signature envelope.

Verify

use K2gl\Dsse\Envelope;
use K2gl\Dsse\EcdsaP256Verifier;
use K2gl\Dsse\Exception\SignatureVerificationFailed;

$envelope = Envelope::fromJson($json);

try {
    $payload = $envelope->verify(EcdsaP256Verifier::fromPem($publicKeyPem));
    // $payload === 'hello world'
} catch (SignatureVerificationFailed) {
    // no signature matched any supplied verifier
}

The envelope is accepted if any signature verifies against any verifier you pass, mirroring the spec's verification model. Pass several verifiers to accept a set of trusted keys.

Ed25519

use K2gl\Dsse\Ed25519Signer;
use K2gl\Dsse\Ed25519Verifier;

$keypair  = sodium_crypto_sign_keypair();
$signer   = new Ed25519Signer(sodium_crypto_sign_secretkey($keypair), 'ed-1');
$verifier = new Ed25519Verifier(sodium_crypto_sign_publickey($keypair));

ECDSA P-384 / P-521 and RSA

The other bundled algorithms follow the same fromPem() pattern:

use K2gl\Dsse\EcdsaP384Signer;
use K2gl\Dsse\EcdsaP384Verifier;
use K2gl\Dsse\RsaSigner;
use K2gl\Dsse\RsaVerifier;

$signer   = EcdsaP384Signer::fromPem($p384PrivateKeyPem);
$verifier = EcdsaP384Verifier::fromPem($p384PublicKeyPem);

// RSASSA-PKCS1-v1_5; hash algorithm defaults to sha256 (sha384/sha512 also supported)
$rsaSigner   = RsaSigner::fromPem($rsaPrivateKeyPem, hashAlgorithm: 'sha512');
$rsaVerifier = RsaVerifier::fromPem($rsaPublicKeyPem, hashAlgorithm: 'sha512');

// RSASSA-PSS (MGF1, salt as long as the hash) — what the Go and Python reference
// implementations sign with; same hash choices
$pssSigner   = RsaPssSigner::fromPem($rsaPrivateKeyPem);
$pssVerifier = RsaPssVerifier::fromPem($rsaPublicKeyPem);

EcdsaP521Signer / EcdsaP521Verifier work identically. ext-openssl has no PSS padding, so RsaPssSigner / RsaPssVerifier run the RSA primitive bare and do EMSA-PSS (RFC 8017) themselves — no extra dependency.

The ECDSA signers write raw r||s by default; ask for DER when the consumer is one of the reference implementations, which verify only that:

use K2gl\Dsse\SignatureEncoding;

$signer = EcdsaP256Signer::fromPem($privateKeyPem, encoding: SignatureEncoding::Der);

Loading a key without knowing its algorithm

When a key arrives as a PEM file or a JWK from a JWKS endpoint, PublicKey picks the right verifier for you — RSA, ECDSA (P-256/384/521) or Ed25519 — so you don't have to branch on the algorithm yourself:

use K2gl\Dsse\PublicKey;

$verifier = PublicKey::fromPem($publicKeyPem); // detects the algorithm and curve
$verifier = PublicKey::fromJwk($jwk);          // EC / RSA / OKP (Ed25519)

$payload = $envelope->verify($verifier);

An RSA PEM carries neither hash nor padding, so it verifies PKCS#1 v1.5 over SHA-256; for another hash use RsaVerifier::fromPem($pem, hashAlgorithm: 'sha512'), for PSS RsaPssVerifier. An RSA JWK usually says in alg what it is for, and that is honoured: RS256/RS384/RS512 pick the hash, PS256/PS384/PS512 pick PSS.

KeyId computes the two identifiers commonly used for a signature's keyId:

use K2gl\Dsse\KeyId;

KeyId::sha256Spki($publicKeyPem); // hex SHA-256 of the DER key (cosign / Sigstore style)
KeyId::jwkThumbprint($jwk);       // RFC 7638 base64url thumbprint

Plugging in your own key backend

Implement two methods to sign with a KMS/HSM or any other scheme:

use K2gl\Dsse\Signer;

final class KmsSigner implements Signer
{
    public function sign(string $message): string { /* sign PAE bytes, return raw signature */ }
    public function keyId(): ?string { return 'arn:aws:kms:...'; }
}

Interoperability

Envelopes made by the two reference implementations — go-securesystemslib and securesystemslib (Python) — are part of the test suite, one per scheme: ECDSA P-256/384/521, Ed25519, RSASSA-PSS and, from Python, RSASSA-PKCS1-v1_5. The vectors under tests/fixtures/interop were produced by the generators in tests/interop, and CI regenerates them with the upstream code on every run and checks the other direction too: envelopes from every bundled signer are verified by Go and by Python.

Two things the vectors settled. The reference implementations sign RSA with PSS (Go has nothing else), hence RsaPssSigner / RsaPssVerifier. And they verify ECDSA signatures in DER only, hence SignatureEncoding::Der on the ECDSA signers; the verifiers here take either form.

Design

• Crypto-agnostic core. Pae and Envelope carry no cryptography; signing is delegated to Signer / Verifier, so you control the algorithm and key storage.
• Raw signatures by default. The bundled ECDSA signers emit raw r||s signatures (64/96/132 bytes for P-256/384/521), the form JOSE and WebCrypto use, converting from OpenSSL's DER internally; SignatureEncoding::Der keeps the DER for consumers that want it (Sigstore bundles, the Go and Python reference implementations). The verifiers accept both, detecting the encoding automatically.
• Strict and typed. declare(strict_types=1) throughout, analysed at PHPStan level 9; every exception implements DsseException.

License

MIT — see LICENSE.

Based on the DSSE specification (Apache-2.0) by the Secure Systems Lab; this is an independent, clean-room PHP implementation.